Cerium-containing oxide nanoparticles have many current industrial uses, along with many emerging technical applications. They are well known as important components, for example, in three-way automotive exhaust catalysts, automotive fuel borne catalysts, water gas shift reaction catalysts, polishing and planarization agents, solid oxide fuel cells, hybrid solar cells and ultra-violet sun blockers. There are many synthetic processes for the production of metal oxides, including aqueous and hydrothermal precipitation, spray precipitation, combustion, plasma deposition and electrochemical techniques, among others. While a variety of solvents may be used in these synthetic processes, aqueous reaction chemistries are particularly favored in manufacturing processes where high material through-put is desired. However, conventional aqueous processes—precipitation in particular—are costly as they involve multiple steps that are often time and energy consuming, as well as equipment intensive.
Conventional large-scale metal oxide manufacturing processes can typically be divided into three stages: aqueous precipitation of precursor compounds, calcination to promote chemical reaction and to enhance crystallinity, followed by final particle size adjustment. In more detail, aqueous precipitation includes the initial steps of reactant delivery, reactant dispersal, particle precipitation, isolation, washing, drying, and optional impregnation with other metal ions; calcination involves heating to 400-1000° C. for several hours; followed by grinding, milling or classification to adjust the final particle size, among other steps.
One approach to reduce the number of steps in the aqueous preparation is to employ methods that produce a stable aqueous dispersion (suspension, colloid, sol) of the final particles directly from the initial reactants, thereby avoiding the time, cost and potential contamination inherent in the particle precipitation, isolation, and drying steps. Moreover, if the particles produced in such a direct method are sufficiently pure, wherein the chemical composition of the particles is as desired, and the particles are sufficiently crystalline, then the calcination step may also be eliminated. In addition, if the particle size and size distribution produced by such a direct method are substantially as desired, then the grinding, milling and classification steps may also be eliminated. Direct methods to produce aqueous dispersions (suspensions, colloids, sols) of crystalline cerium-containing oxide nanoparticles without the use of precipitation, isolation, drying, calcination, grinding, milling or classification steps, and the like, are described in commonly assigned U.S. patent application Ser. No. 12/779,602, now Publication US 2010/0242342 A1, by A. G. DiFrancesco et al. The '342 reference discloses stable aqueous dispersions of crystalline cerium-containing nanoparticles in a size range, for example, of 1-5 nanometers.
While substantial progress has been made in eliminating manufacturing steps from the synthetic process by which stable aqueous dispersions of metal oxide nanoparticles are prepared, use of these nanoparticles in applications such as fuel-borne combustion catalysts requires that dispersions of these nanoparticles also exhibit colloidal stability in the fuel. Such stability would also be required for a fuel additive, miscible in the fuel. Thus, these particles, although readily formed and suspended in a highly polar aqueous phase, must then be transferred to a substantially non-polar phase, a process known as solvent shifting. This problem is conventionally addressed by the use of particle stabilizers. However, most particle stabilizers used to prevent particle agglomeration in an aqueous environment are ill-suited to the task of stabilization in a non-polar environment. When placed in a non-polar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable particulate properties. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation and re-dispersal methods such as, for example, precipitation and subsequent re-dispersal with a new stabilizer using, for instance, a ball milling process, which can take several days and tends to produce polydisperse size frequency distributions.
One approach to simplifying the solvent shifting process employs diafiltration methods and glycol ether solvents having a polarity intermediate between that of water and those of non-polar hydrocarbons. The intermediate polarity colloid is then further shifted to reduce the polarity of the cerium-containing nanoparticle dispersion, as disclosed in commonly assigned U.S. patent application Ser. No. 12/549,776, now Publication US 2010/0152077A1 to Alston et al. Diafiltration, sometimes referred to as cross-flow microfiltration, is a tangential flow filtration method that employs a bulk solvent flow that is tangential to a semi-permeable membrane. However, drawbacks of diafiltration methods include the following: relatively slow filtration rates, substantial financial investment in equipment (e.g. pumps and microfilters), and production of a relatively large amount (e.g. several turnover volumes) of waste solvent.
Use of promoter agents to accelerate transfer of iron oxide nanoparticles from aqueous to non-polar solvents is known in the art. U.S. Pat. No. 7,459,484 to Blanchard et al. discloses use of promoter materials having alcohol functionality and having 6 to 12 carbon atoms to promote transfer, and to improve stability of the organic colloid so formed. US Patent Application Publication 2006/0005465 A1 to Blanchard et al. discloses contact of basic aqueous colloids of rare earth or mixed rare earth/other oxide nanoparticles with an acid and a diluent to form an organic colloid dispersion. U.S. Pat. No. 6,271,269 to Chane-Ching et al. discloses direct transfer of cerium oxide or doped cerium oxide colloidal particles from a counterpart aqueous dispersion. Use of alcohol-based promoters is disclosed as well. However, high process temperatures and times for the transfer of the colloidal particulates represent a significant limitation of the prior art process. It is also apparent that concern over the presence of ionic constituents, and other materials needed to bring about the formation of the colloidal particulate material in the aqueous reaction mixture, affects the viability of the direct process.
Thus, progress has been achieved in reducing the cost of producing and solvent shifting aqueous dispersions of cerium-containing nanoparticles. However, further improvements in manufacturing efficiency are desired, particularly in the case of nanoparticle dispersions used as fuel-borne combustion catalysts that require dispersion stability in both a low-polarity solvent carrier of a fuel additive or in the fuel itself.
It would be very desirable to transfer oxide nanoparticles directly from the aqueous reaction mixture in which the nanoparticles are formed, to a substantially non-polar phase, at low temperatures, to reduce manufacturing hazards in dealing with combustible liquids. At the same time it is desirable that the nanoparticle colloidal dispersions that are the fuel additives exhibit excellent colloidal stability and good fluid flow properties at low ambient temperatures.